In recent years, with the increasing demand for wireless devices for mobile communication, various communication systems have been developed, and a high performance, small, and light-weight wireless device that complies with a plurality of communication systems by an integrated unit is being desired to come out on the market. Accordingly, there is an inevitable demand for the development of antennas equipped in these wireless devices.
Typical example of a device for such mobile communication is the portable telephone system, which is widely used all over the world and the frequency band of which varies depending on the area. As an example, the frequency band used for digital portable telephone system is 810 to 960 MHz in Japan for Personal Digital Cellular 800 (PDC800) system, and in Europe and America, 890 to 960 MHz for Group Special Mobile Community (GSM) system, 1,710 to 1,880 MHz for Personal Communication Network (PCN) system, and 1,850 to 1,990 MHz for Personal Communication System (PCS). As far as the antennas built into the portable telephones conforming to these systems is concerned, planar inverted-F type antennas have been generally and widely used so far. A description will be given on a typical example of such antennas referring to FIG. 26 and FIG. 27.
FIG. 26 is a perspective view of a prior art antenna. FIG. 27 is a partially cut-away perspective view of the rear side of a portable telephone that incorporates the antenna. In FIG. 26, for example, grounding conductor plate 2 made of 0.2 mm thick copper alloy is disposed underneath and in parallel with antenna element 1 made of copper alloy plate having approximate dimensions of 35 mm×45 mm, and 0.2 mm thickness located at a distance of 9 mm from antenna element 1. As shown in FIG. 26, antenna element 1 is secured to grounding conductor plate 2 by means of a support member 1a made of a resin-based dielectric material such as ABS and PPO. First terminal 3 formed on one end of antenna element 1 is electrically connected with grounding conductor plate 2 by soldering and the like method. Antenna 7 is configured in a manner such that second terminal 5 is provided at feeding point 4 near first terminal 3 of antenna element 1 being protruded from grounding conductor plate 2 through hole 6 without any electrical contact with grounding conductor plate 2. On the other hand, as shown in FIG. 27, antenna 7 is disposed inside rear case 9 of portable telephone 8. Though not shown in FIG. 27, grounding conductor plate 2 of antenna 7 is electrically connected with a metal shielding section formed on the inside surface of rear case 9, and second terminal 5 of antenna 7 is electrically connected by press fit and the like method with a radio frequency circuit board disposed inside rear case 9 of portable telephone 8.
A description on the operation of antenna 7 described above and portable telephone 8 employing antenna 7 will now be given in the following.
First terminal 3 formed on antenna element 1 of antenna 7 is an inductive line while the other parts excluding the part of first terminal 3 of antenna element 1 as viewed from feeding point 4 forms a capacitive line. Side lengths L1, L2 of antenna element 1, width L3 of first terminal 3, and distance L4 between first terminal 3 and feeding point 4 are so determined that the input impedance of antenna 7 in a desired frequency band as viewed from feeding point 4 of antenna element 1 will give a desired value. The input impedance is determined by the position of feeding point 4, namely L3 and L4, and the impedance matching with the input/output impedance of 50Ω of the radio frequency circuit can be obtained in a desired frequency band. When transmitting or receiving with portable telephone 8, the signal power as transmitted or received in a desired frequency band by antenna element 1 is put out from or supplied to the radio frequency circuit placed in rear case 9 of portable telephone 8 through second terminal 5 formed on antenna element 1, respectively. Technical details of such a planar inverted-F type antenna are published in “New Antenna Engineering” (in Japanese), ISBN4-915449-80-7, pages 109-114, and many other technical papers and books. According to these literatures, the planar inverted-F type antenna is suitable as an antenna for portable telephones that require a small size, high gain, and wide directional radiation pattern. It gives an advantage of not only enabling relative downsizing and slimming for incorporation into the case of a device but also providing freedom of device design. There is also an advantage that, by built-in constitution of the antenna, the antenna is better protected from mechanical shocks than a non-built-in antenna, and the antenna will scarcely experience mechanical damage thereby lengthening life of the antenna.
However, the operating frequency band, being a key factor of electrical characteristics, of these prior art antennas has only a specific bandwidth of approximately 3% at the maximum. The only way to improve this is to enlarge the shape, which will make the antenna inappropriate for use as a small, thin, wide-band, and high sensitivity built-in type antenna that is demanded by the market. Also, even though wide bandwidth and high sensitivity are pursued at the expense of miniaturization, a complicated impedance matching circuit will be required between the antenna and the radio frequency circuit thus presenting an obstacle for price reduction of portable telephones.